Controlling re-charge of a nickel metal-hydride (NiMH) or nickel cadmuim (NiCd) battery

ABSTRACT

A method of charging a NiMH or NiCd battery such that charging does not occupy a substantial amount of time and such that the operating temperature of the battery is reduced during the lifetime of the battery, thereby extending the life of the battery. This invention is a method of charging that is accomplished using a temperature differential method to continuously regulate the rate of charge injected into the battery. An amplifier is used to amplify the difference in temperature between one or more cases in which the battery is contained and ambient temperature. In this method, the greater the difference in temperature, the lower the charging current is injected into the battery. The method of charging may be incorporated into uninterruptible power supply equipment.

CLAIM OF PRIORITY TO PRIOR APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 09/838,753, filed Apr. 19, 2001 now abandoned, the disclosureof which is incorporated herein by reference.

FIELD OF THE INVENTION

The present application relates generally to a system containingrechargeable batteries and a method for recharging them when they becomedischarged. More specifically, the present application relates to anovel method of recharging NiMH and NiCd Batteries and maintaining theirstate of charge between discharges. Such batteries may be incorporatedinto an uninterruptible power supply (UPS) to ensure power availabilityfor critical and/or sensitive loads, as disclosed with respect to NiMHbatteries in application Ser. No. 09/838,753.

BACKGROUND OF THE INVENTION

When a NiCd battery or NiMH battery is fully charged, any additionalcharge current is converted to heat within the cell and needs to bedissipated in order to keep reduce the damaging effects of hightemperature on the battery's service life. The most common method foundin many low cost chargers is to inject current into the battery(s) at avery low rate. The benefit of this method is that once the battery isfully charged, the minimal amount of heat generated by the continuousovercharge current can be adequately dissipated through the cell wallsand the cell's temperature is not high enough to cause serious damage tothe battery. The adverse affect of this method is that it takes manyhours (e.g. 20) to recharge the cells to 100% state of charge (SOC).Another problem is that in some climates, the self-discharge rate may behigher than the charge rate from the charger. This results in ano-charging condition or where the battery(s) charges to a finitepercentage below 100% SOC or even discharges to zero % SOC.

To cause a faster recharge time, some “fast” chargers are availablewhich inject higher current, but the charge period is terminated byeither user intervention (as required by user manuals, quoting “beforepermanent damage to the battery(s) is incurred.”) or by a timermeasuring a fixed time interval from the start of charge. In eithercase, more times than not, the battery(s) is either left charging toolong or too short resulting in damaged or undercharged battery(s).

A more sophisticated approach used by higher cost systems is to injectcharge at a higher rate, but stop the high-rate charge automaticallyafter an end-of-charge indication is sensed. In NiMH and NiCd batteries,as the 100% SOC charge condition is approached, the temperature andterminal voltage rise rapidly. Therefore, the end-of-charge conditioncan be sensed by the charger measuring the battery(s) terminalvoltage(s), the rate of change of terminal voltage or temperaturelevels. However, voltage and temperature levels and their rates ofchange are determined by not only the battery's state of charge but theambient temperature and charge rates as well. The higher the rate ofcharge and ambient temperature, the higher the battery's casetemperature and its terminal voltage will be at any state of charge. Itbecomes a daunting task to program a smart charger to take into accountall of the environmental and forcing functions to determine the proper100% SOC point and terminate fast charge for optimum batteryperformance.

The use of UPS systems having battery backup systems to provideregulated, uninterruptible power for such equipment as computer systemsis well known. Typically, most UPS systems use some type of lead acidbattery to provide backup power. Lead batteries, however, haveperformance limitations especially when they are discharged at rateswell above their specification rates or when they are operated attemperature extremes. NiMH and NiCd battery chemistries provideadvantages when used in UPS systems, detailed below.

While the invention is disclosed as being useful in the charging of NiMHand NiCd batteries in a UPS system, it is understood that it may be usedin many other environments, for example, as a standalone charging deviceor used in other equipment and devices which contain such batterieswould periodically require recharging.

SUMMARY OF THE INVENTION

It is desirable to have a method of charging a NiMH or NiCd battery suchthat charging does not occupy a substantial amount of time and such thatthe overall operating temperature of the battery during use andespecially charging is reduced during the lifetime of the battery,thereby extending the life of the battery. This invention is directed toa method of charging that is accomplished using a temperaturedifferential method to continuously regulate the rate of charge injectedinto the battery. An amplifier is used to amplify the detected or senseddifference in temperature between the one or more cases in which thebattery or batteries are contained and ambient temperature. In thismethod, the greater the difference in temperature, the lower thecharging current is injected into the battery.

In one general aspect, the invention features a power supply systemincluding a power input to receive input power from a power source, apower output to provide output power to the battery(s), at least oneNiMH or NiCd battery, a charger coupled to the power input and thebattery to convert input power and output it in a regulated manner tothe battery, a controller, coupled to the charger, constructed andarranged to monitor and control the power from the charger into thebattery.

In general, in another aspect, the invention is directed to a method ofcharging a battery. The method of charging comprises detecting at leastone battery, sensing a first temperature at a point proximal to thebattery, sensing a second temperature at a point distal to the battery,and controlling current into the battery according to a differencebetween the first temperature and the second temperature. The inventionprovides one or more of the following advantages. The charging method ofthe invention improves the electrical efficiency of the charge processas the battery is charged to a 100% state of charge as quickly as thebattery can accept charge. One hundred percent SOC is maintained in thebattery without overheating the cells of the battery and by matching therate of self-discharge with changes based on the battery age and ambientconditions. The consistency of charge rates is increased withoutcomputations with a microprocessor or other digital means. Eliminatingthe need for microprocessors in chargers implementing the batterycharging method reduces the cost associated with a charger. Further,charging batteries according to the method of the invention increasessafety by reducing a dependency on complex algorithms using voltage,temperature, and/or rate of charge of the batteries.

An uninterruptible power supply (USP) system used with the chargingsystem of the present application includes one or more NiMH or NiCdbatteries. NiMH and NiCd battery chemistries are desirable because theyare respectively about 2 and 5 times volumetrically and gravimetricallymore energy dense than an equivalent lead acid battery that is typicallyused in a UPS. Thus, the size of an NiMH or NiCd battery is much smallerand lighter than a similarly performing lead acid battery and makes theproduct into which it is installed more attractive, versatile and usefulto customers. The extra volume required by similarly performing leadacid batteries typically requires extra floor space and costly hardwareto install the batteries.

Another advantage of both NiMH and NiCd batteries is that they arerelatively temperature immune. Their performance suffers little at theextremes of the lead acid limits, and their lifetime is not affected asdramatically by temperature as are lead acid batteries. It has beenestimated that NiMH battery life times are around 10–15 years. Lead acidbatteries can be designed to last similarly long, but compromise theirenergy density doing so.

The invention will be more fully understood after a review of thefollowing figures, detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the figures, which are incorporated herein by reference and in which:

FIG. 1 is a block diagram of a system having a NiMH or a NiCd battery inaccordance with one embodiment of the invention;

FIG. 2 is a diagram of a battery charging device for a battery inaccordance with one embodiment of the invention;

FIG. 3 is a temperature compensating circuit in accordance with oneembodiment of the invention;

FIG. 4 is a diagram showing an amplifier used in the charging controlmechanism in accordance with one embodiment of the invention; and

FIG. 5 is a graph illustrating the relationship between current andtemperature in the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of the present invention is directed to a system thatincludes one or more NiMH or NiCd batteries and a method for chargingsuch batteries. An embodiment of the invention provides a method ofdetermining the rate at which to charge a battery using a temperaturedifferential, rather than an absolute temperature, absolute voltage, orrates thereof to determine rate of charge. Other embodiments are withinthe scope of the invention.

FIG. 1 shows an exemplary system 10 used to provide battery power inwhich a NiMH or NiCd battery can be used. The system 10 includes a powerinput 14, a charger 15, a controller 16, and a NiMH or NiCd battery 18.The power input 14 can include, for example, a circuit breaker or othercurrent-limiting device, and/or a filter and/or a rectifier. The charger15 can include, for example, one or more power-control switch(es), whichare controlled by the controller 16.

The system can also include a current meter 119 or other currentmeasuring means to measure a current of the battery, such as, forexample, an ammeter, a Hall-effect sensor device or a voltmeter 119 thatmeasures the voltage across a current shunt 117, and a temperaturemeasuring device 121.

The exemplary system 10 can operate as follows. The circuitbreaker/filter 112 receives input AC or DC power from the power sourcethrough the input 14, filters the input power and provides filteredpower to the charger 15. The charger converts the filtered power to DCpower having a predefined voltage value depending on the number and typeof batteries being charged and at a current rate specified by thecontroller 16 and charge battery or batteries 18. It is understood bythose skilled in the art that the system 10 may be incorporated intoknown UPS systems to provide a charging system for one or more NiMH orNiCd batteries incorporated into such UPS systems.

Regardless of the size or kind of battery being charged, the temperaturedifferential charging method achieves a substantially consistent andpredictable battery reaction. The temperature differential chargingmethod controls the charging current of a battery by measuring the heatgenerated inside the battery and controlling the current into thebattery based on the measured heat. Current is regulated between zeroand a fixed maximum according to the inverse proportionality of the heatgenerated inside the battery. The heat is an indicator of the state ofcharge of the battery. Current is measured and controlled based on theheat, and the battery is maintained at 100% state of charge. Minimizingthe battery's internal temperature, however, will prolong battery life.Thus, a control mechanism is employed to strike a balance between theheat generated and prolonged battery life.

The temperature differential charging method employs the principle thatextra charge not contributing to a battery's state of charge is turnedinto heat. Even before 100% state of charge is reached, some of thecurrent is turned into heat, while some is converted to stored energy.Additional current causes extra heat, which is not only wasteful, but isdamaging to the battery. The optimal amount of charge current is enoughto charge the battery to 100% or nearly so without heating it up.Therefore, the best way to determine the optimum rate of charge is tomeasure the amount of heat produced on the charging process and controlthe charge current to regulate an amount of waste heat to apredetermined and small level.

The control mechanism of the present invention balances the heatgenerated and the life of the battery, discussed below, and does so byperforming a heat flow measurement. Referring to FIG. 2, placement oftemperature sensors to determine heat flow from a battery is shown. Abattery charger 250 with temperature compensating function includescharger circuitry of a known type 210, a power (AC or DC) input 212, abattery 214, a first temperature sensor 216, a second temperature sensor218, a thermal insulator 220, charger adjustment input 222, andtemperature compensating circuitry 224. Charger power converters havingthe charging circuitry 210 can be of any known type to convert the inputpower 212 to inject current into the battery 214 for the purpose ofmaintaining the battery at full charge. The charger circuitry 210 canalso control the output current based on input from the battery terminalvoltage and the output current adjustment from the temperaturecompensating circuitry 224. The temperature compensating circuitry 224can adjust the charger circuitry 210 output current based on the batterytemperature measured at the first temperature sensor 216, and ambienttemperature measured at the second temperature sensor 218, and thecharger adjustment input 222. The first sensor 216 may be attached bysuitable means (not shown) directly onto the battery case of the batteryto sense the battery temperature, or may be spaced from but remain closeto the battery case. The second sensor 218 is preferably located somedistance from the battery case 214 so that the second sensor isrelatively unaffected by the heat of the battery and can more accuratelymeasure ambient temperature.

The battery can be any series or parallel combination of NiCAD or NiMHbatteries. The input power can be AC or DC of any voltage. The chargeradjustment input can be from any analog adjustment such as a manually orautomatically set potentiometer or voltage source, programmableresistance, microprocessor-controlled signals, or any other of a numberof known ways to provide overriding adjustment to the output chargingcurrent. The charger adjustment input allows for calibration of theoutput current to compensate for circuit parameter variations.

The first and second temperature sensors 216 and 218 can be temperaturesensitive resistors, diodes, thermocouples, or any other device whoseparameter(s) change with temperature. The thermal insulator 220 can be aplastic insulator, a gap of air, a glass mat, paper, or any otherthermal barrier that allows a temperature differential to exist betweenthe first sensor 216 and the second sensor 218 when a net thermal energyis dissipated by the battery, shown by arrows 226. Thus, the thermalinsulator may be designed such that it isolates second sensor 218 fromthe heat 226 emanating from the battery 214.

In FIG. 3, an exemplary temperature compensating circuit 224 is shown inmore detail. The temperature compensating circuit includes diode strings260 and 262. The voltage of a diode decreases with increasingtemperature. Thus, the voltage at point 282 decreases when thetemperature around diode string 262 increases. A decreasing voltage atjunction 282 causes the output voltage at output 278 to decrease,thereby decreasing the charger current output. If diode string 262 isplaced near the battery when the battery starts to warm up near the endof a charge cycle, the charge current will decrease. The decreasingcharge current will reduce the temperature of the battery and the chargecurrent will stabilize at a point in which a nominal amount of heat isgenerated inside the battery due to a small amount of excess currentgoing into the battery.

A resistor 264 creates a small offset in the voltage between points 282and 274 which allows the temperature of the diode string 262 to beslightly higher than that of the diode string 260 to ensure that thereis always some charging current even when the battery is fully charged.An op-amp 266 balances the current between diode strings 262 and 260.Feedback elements 280 compensate the overall control loop to maintainstability. An adjust-in input 272 adjusts offsets due to parametricvariations in the op-amp 266 and in op-amp 276, resistors 268 and 270,and the current/voltage relationship between diode strings 260 and 262.

An error-out signal 278 connects to the output current adjust of thecharger circuitry 210, shown in FIG. 2. The polarity of this signal canbe modified to suit any standard required by the charger circuitry 210.For example, a higher voltage at the error output 278 equates to arequest for more current into the battery. Switching the inputs to theop-amp 266, however, will cause the reverse to occur, i.e., lowervoltage on output 278 causes the charger current to increase. Theselection of signal polarity and level is arbitrary and not part of thenovelty of the invention.

The diode strings 260 and 262 are temperature sensors in that theirrespective current/voltage relationships change with temperature. Anytemperature sensitive device can be used. Depending on the polarity ofthe temperature dependency, the input op-amp 276 can be reversed inorder to properly compensate for a temperature differential. If thebattery temperature increases, the error output signal 278 increasessuch that the charger current is reduced.

In the temperature compensating circuit of FIG. 3, a plurality oftemperature sensors in diode string 262 indicate a plurality ofbatteries whose temperatures are being sensed. In other embodiments,however, a lesser number of temperature sensors are used than there arebatteries. Further, an equal number of temperature sensors are includedin string diode 260. String diode 260 measures ambient temperature sothat a differential temperature measurement can be achieved. In otherembodiments, the ambient temperature is measured using any other knownmeans, for example, a single diode drop can be amplified a number oftimes by an amplifier stage. Other temperature dependent elements aresuitable, including but not limited to, thermistors and thermocouples.

The control mechanism works according to an algorithm. Referring againto FIG. 2, to achieve an appropriate level of charge for the battery, aheat flow measurement from the battery is taken. As heat is generated inthe battery, heat flows from the battery to the ambient at a rate shownby arrows 226 and defined as P_(diss). This is called the energydissipation rate. Heat will flow through all barriers and cause thetemperatures closer to the battery to be higher than those further away.The higher the thermal resistance of the barriers in the path of thisheat, the higher the temperature differential will be across thebarrier. The relationship between ΔT and P_(diss) is:ΔT=P _(diss) ×Z _(th)  (1)The greater the thermal resistance, Z_(th), the greater the temperaturedifferential for any given energy dissipation rate.

An amplifier used in the control mechanism for charging the battery anddiscussed in detail with respect to FIG. 3, is shown in FIG. 4 forsimplicity. Referring to FIG. 4, the control mechanism 200 includes anamplifier 202 with a high gain, Ao (>100,000), a current regulator 204and a battery 206 to be charged. A signal input into the amplifier 202,ΔT, is a signal of the magnitude of the difference in temperatures T1and T2 between the battery case and that of a spot somewhere between thebattery and ambient, and is obtained from the apparatus shown in FIG. 3.The other signal into the amplifier is a reference signal, Td,representative of the ideal ΔT to maintain between the spots where thetemperature is measured. The relationship between the charge current Io,Td and ΔT are shown in FIG. 5.

Graph 1 shows that the higher ΔT is, the lower the charging current. Thelower the ΔT is, the higher the charging current until a maximum levelfixed by either the natural upper capabilities of the charger, or by anartificial limit imposed by design to protect the batteries ifnecessary. The slope of the transition from full charging current tominimum charging current is set by the open loop gain of the amplifierin FIG. 4, Ao. On the slope between the minimum and maximum levels, therelationship between Io, Td and ΔT is:Io=Ao×(Td−ΔT)  (2)

When the charger outputs current into the battery, there are twocomponents of this current. The current that is directly converted tostored chemical energy is represented by Ichg. The current not convertedto chemical energy contributing to a peripheral reaction that generateswaste heat is represented by Iovc.

When the battery is depleted, almost all of the current is Ichg (evenwhen empty, some of the current does not turn into chemical energycausing Iovc to never be zero). When the battery is fully charged,almost all of the current is Iovc (some internal self discharge happenswhich causes Ichg to never go to zero). But in any state,Io=Ichg+Iovc  (3)

Inside the battery there are two mechanisms that generate heat.“Friction” heat generated by electrons (electrical current) bumping intometallic atoms is a first mechanism. This heat is quantified by:Io²×Rs,  (4)

where Io is the total charging current, and Rs is the electricalresistance of the conductors in the battery.

The second mechanism, reaction heat generated by a peripheralelectrochemical reaction due to excess electrons, is quantified by:Iovc×Vb,  (5)

where Vb is the battery terminal voltage.

The total energy dissipation rate from the battery is the sum of thesetwo quantities:Pdiss=(Io ² ×Rs)+(Iocv×Vb)  (6)Combining equation (6) with equation (1) above and solving for ΔTresults inΔT=(Io ² ×Rs×Zth)+(Iocv×Vb×Zth)  (7)This is a relationship between the charging current, battery parametersand the measured temperature differential at the two probes.Control Loop Equations

To find the output current to which the control mechanism will regulatebased on the ΔT measured, ΔT is substituted from equation (7) intoequation (2) to produce:Io=Ao×[Td−(Io ² ×Rs×Zth)−(Iocv×Vb×Zth)]  (8)Io, appearing on both sides of the equation, is a function of itself.This is a relationship of a control system with feedback.

The amplifier in FIG. 4 has a very high gain, Ao. Thus, the right sideof equation (8) is so much higher than the left that the left side canbe approximated as zero. For example, a practical value of chargecurrent may be 1 Amp in a fast charger. Dividing both sides, by Ao(>100,000) the left side is a very small number (1/100,000) and theright side is a number much closer to one. Therefore, if Ao is verylarge as it is in a practical amplifier circuit, equation (8) can beclosely approximated by:0=Td−(Io ² ×Rs×Zth)−(Iocv×Vb×Zth)  (9)Solving for Td shows that Td has two components:Td=(Io ² ×Rs×Zth)+(Iocv×Vb×Zth)  (10)The first term, Td, is the component due to the total current passingthough resistance Rs electrically causing heat and the second term isthe component due to the excessive charge current chemically causingheat.Controlling the Charge Current During the Entire Recharge Cycle

Throughout the charging cycle from depleted to full, the chargingcurrents vary and the contributions to the heat generation switch frommostly electrical to mostly electrochemical. The simple circuits inFIGS. 2–4 maintain a temperature differential across the two temperatureprobes by controlling the total current fed into the battery. The higherthe temperature differential, the more that the control circuitthrottles back on the current. So, whether the heat generated frominternal Io²Rs losses, or from electrochemical reactions, the totalenergy dissipated is maintained constant regardless of the state ofcharge of the battery.

When the battery is depleted, the overcharge current Iocv is very low,so the dominant factor in terms of the heat generations is the firstterm of equation (10) because the second term is reduced to a smallnumber:Td=(Io ² ×Rs×Zth)  (11)Solving for Io yields:

$\begin{matrix}{{Io} = \sqrt{\frac{Td}{{Rs} \times {Zth}}}} & (12)\end{matrix}$In equation (12), Io is the total charging current when the battery isfully discharged.

As the battery becomes more charged, Iovc starts to increase and Ichgbegins to drop to near zero. This is a naturally occurring phenomenon inthe battery during the course of the recharge cycle. Toward the end ofthe recharge cycle and beyond, the dominant factor in heat generation isthe second term of equation (10):Td=(Iocv×Vb×Zth)  (13)Solving for Iocv produces the relationship:

$\begin{matrix}{{Iocv} = \frac{Td}{{Vb} \times {Zth}}} & (14)\end{matrix}$In equation (14), Iocv is the overcharge current producing waste heat inthe battery.

Ichg at the end of the charge cycle is very small but equal to theself-discharge rate of the battery. It is not insignificant compared toIovc. The total current into the battery Io is found by adding these twoterms.

In a battery, electrons leak from one electrode to the other and thebattery discharges itself internally. Some batteries leak more thanothers and this leakage rate is proportional to the battery temperatureand other factors. To compensate for this effect, some chargers fix asmall amount of current to balance this effect. Too much current causesexcessive electrons to produce heat-generating chemical reaction, whiletoo little will allow the battery to discharge below 100%state-of-charge. Maintaining 100% state-of-charge of a fully chargedbattery is commonly called “float charging.”

It is known that excessive current causes heat, and thus the optimalfloat charging current can be achieved by regulating the heat flow fromthe battery to a minimal level. The heat flow can be detected by atemperature differential across two probes as shown in FIG. 3. If a zerotemperature differential is detected, the battery is not being charged.If too high a temperature differential is detected, the battery is beingcharged too much. Finding the balance is key to keeping the battery at100% state of charge vs. overheating the battery, which causes itsservice life to be shortened.

The control circuit is to reduce Iocv to such a level that the internaltemperatures are as close to ambient as possible. This value is set byfixing the control reference signal, Td, in FIG. 4. A value for Td thatis too small will reduce the charging current when the battery is empty,which would cause the battery to take too long to charge from a fullydischarged state.

EXAMPLE

By way of example if,

-   -   Vb is 1.5V (typical for one cell),    -   Rs is 0.1 Ω (typical of a small battery size),    -   Zth is 10° C./Watt (a function of the placement of the probes)    -   Set Td to 0.2° C. (by controlling the reference voltage to the        amplifier).

To determine the steady-state overcharge charge maintenance current, useequation (14):

$\begin{matrix}{{Iocv} = {\frac{Td}{{Vb} \times {Zth}} = {13\mspace{14mu}{mA}}}} & (14)\end{matrix}$and according to equation (6),Pdiss=(Io ² ×Rs)+(Iocv×Vb)=20 mW  (6)20 mW of heat is generated by the excessive charge current in thebattery during the float charging mode. This is a small and manageableamount of heat resulting in only a 0.2° C. rise in the battery aboveambient.

To determine the full charging current at the beginning of the chargecycle, use (12):

$\begin{matrix}{{Io} = {\sqrt{\frac{Td}{{Rs} \times {Zth}}} = {0.45\mspace{14mu} A}}} & (12)\end{matrix}$and the internal Pdiss is also,Pdiss=(Io ² ×Rs)+(Iocv×Vb)=20 mW  (6)

By fixing the measured temperature differential, ΔT, to Td, the internalenergy dissipation rate is held to a fixed value no matter what thebattery's state of charge. 0.45A will charge a 2 amp-hour battery inapproximately 4 hours.

Thus, in order to maximize service life of a battery, a minimumtemperature is desired. In order to maintain 100% state of charge,however, some level of charge maintenance current is required. Since itis known that the charge current not contributing to state-of-chargemaintenance is converted to heat, a simple and reliable charging methodis to govern the charge current based on a measure of heat emanatingfrom the battery. It is shown that the same mechanism to control floatcharging can be used to control recharge current and achieve relativelyshort recharge times of completely discharged batteries.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications and improvements willreadily occur to those skilled in the art. Such alterations,modifications and improvements are intended to be within the scope andspirit of the invention. Accordingly, the foregoing description is byway of example only and is not intended as limiting. The invention'slimit is defined only in the following claims and the equivalentsthereto.

1. A method of charging a battery comprising: providing at least one ofa Nickel Metal Hydride battery or a Nickel Cadmium battery; sensing afirst temperature at a point proximal to the at least one Nickel MetalHydride battery or the Nickel Cadmium battery; sensing a secondtemperature at a point distal to the battery; and controlling a chargingcurrent by increasing or decreasing a flow of the charging current intothe battery according to a difference between the first temperature andthe second temperature.
 2. The method of claim 1, wherein the at leastone battery is incorporated into an uninterruptible power supply.